Gas Chromatography is a means of chemical analysis used for separating chemicals in a complex sample. A gas chromatograph uses a narrow tube, known as a column, through which different chemical constituents of a sample pass in a gas stream. The gas stream is also called the carrier gas or mobile phase. Gas Liquid Chromatography (GLC), or simply Gas Chromatography (GC), is a type of chromatography in which the mobile phase is a gas. The mobile phase (i.e., a gas stream) is passed through a column with a phase coating, which is called the stationary phase and which is a microscopic layer of liquid on an inert solid wall of the column. The column is often flexible so that a very long column can be wound into a small coil. As the chemical constituents exit the end of the column, they are detected and identified electronically.
In GC, the chemical constituents within a sample pass through the column at different rates, depending on their various chemical and physical properties and their interaction with a specific column filling. Chemical constituents are characterized by the different rates by which they flow through, or elute from, the column. This rate corresponds with the constituents' individual retention time (i.e., the length of time between the injection of the sample and the detection of the individual component). In many cases, constituents with different boiling points have different retention times. Other parameters can also be used to alter the order or retention time, such as the carrier gas flow rate.
The column(s) in a GC are contained in an oven, the temperature of which is precisely controlled (e.g., electronically). The rate at which a sample passes through the column is directly proportional to the temperature of the column.
One application of GC is for the analysis of crude oil. Crude oil, for example, may contain a mixture of chemical compounds from a family of several hundred chemical compounds. Some chemicals that may be found in crude oil include hexane, jet fuels, mineral oils, benzene, toluene, xylenes, naphthalene, and fluorine, in addition to other petroleum products and gasoline components.
One useful tool for analyzing crude oil is its boiling point (BP) curve, as boiling point range is often a key controlling parameter for manufacturing petroleum products. GC is routinely employed for determining the boiling point range of oil products, as it is a fast and inexpensive method for providing information regarding the fuel-type present. A BP curve is a quantitative percent mass yield as a function of the boiling point of the hydrocarbon components of a sample.
However, in the case of crude oil, two column systems are often required since the crude oil BP curve obtained by high temperature columns is not accurate in the region of Initial Boiling Point up to 25% of the BP curve, generally comprising methane through nonane. As a result, a second injection is conventionally made in a separate instrument to analyze the crude oil fraction in the region of methane through nonane. This second injection is often analyzed and identified using a Detailed Hydrocarbon Analysis (DHA) column. The identification on this longer DHA column is often used for providing the boiling point in the region up to 25%. In many cases, the DHA column requires different temperatures than the high temperature columns, thereby preventing both columns from being housed in a single gas chromatograph oven. Generally DHA columns are designed such that they have a limited temperature range and cannot coexist in the same oven as a crude oil column, which is heated to high temperatures.